EP3358662A1

EP3358662A1 - Binder composition, non-aqueous electrolyte secondary battery electrode, and non-aqueous electrolyte secondary battery

Info

Publication number: EP3358662A1
Application number: EP16851148.3A
Authority: EP
Inventors: Shota KOBAYASHI; Kenta Aoki; Hiroshi Sato; Yasuhiro Tada
Original assignee: Kureha Corp
Current assignee: Kureha Corp
Priority date: 2015-09-30
Filing date: 2016-09-13
Publication date: 2018-08-08
Anticipated expiration: 2036-09-13
Also published as: KR102160398B1; HUE052704T2; JP2019075387A; EP3358662A4; EP3358662B1; CN108040504A; WO2017056974A1; CN108040504B; PL3358662T3; KR20180021773A; JP6797223B2; JPWO2017056974A1; US20180269484A1

Abstract

To provide a binder composition having high adhesion, without producing a copolymer or an aggregate of a copolymer and an electrode active material. A binder composition according to the present invention contains: a first vinylidene fluoride polymer with an inherent viscosity of 1.7 dL/g or higher; and a second vinylidene fluoride polymer containing acrylic acid or methacrylic acid as a monomer unit.

Description

TECHNICAL FIELD

The present invention relates to a binder composition, electrode for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery.

BACKGROUND ART

In recent years, development of electronic technology has been remarkable, and high functionality of compact portable apparatuses have been advancing. Therefore, there is demand for power supplies used therein to be smaller and lighter (in other words, higher energy density). Non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries are widely used as batteries having high energy density.
An electrode structure for a non-aqueous electrolyte secondary battery is a structure having a current collector and an electrode mixture layer formed on the current collector. The electrode mixture layer is generally coated on the current collector in a slurry condition where an electrode mixture containing electrode active materials and binders dispersed in an appropriate solvent or dispersing medium, and is formed by volatilizing the solvent or dispersing medium. Electrode active materials or the like serving as cathode or anode active materials are mainly used as electrode active materials for example. A polyvinylidene fluoride or the like is mainly used as the binder.
However, a conventional polyvinylidene fluoride used as a binder has relatively weak adhesion between electrode active materials and current collector, and therefore, phenomena are observed such as the electrode active material shedding during use, the electrode mixture layer peeling from the current collector, and the like. Therefore, the discharge capacity when using a battery over a long period of time may greatly be reduced.
Therefore, several polyvinylidene fluoride serving as a binder with improved adhesion have been developed. A copolymer of acrylic acid and vinylidene fluoride is known as an example of polyvinylidene fluoride (Patent Literature 1). Furthermore, a technique of improving the adhesion of the binder by blending two or more types of a polyvinylidene fluoride is also known (Patent Literature 2). Patent Literature 2 discloses a binder for forming a non-aqueous battery electrode formed by combining a non-modified polyvinylidene fluoride with an inherent viscosity of 1.2 dL/g or higher, and a modified polyvinylidene fluoride having a carboxyl group or epoxy group.

CITATION LIST

Patent Literature

[Patent Literature 1] Japanese Unexamined Patent Application " PCT Application 2010-525124 (Published July 22, 2010 )"
[Patent Literature 2] Japanese Unexamined Patent Application Publication No. " JP H9-320607 A (Published Dec. 12, 1997 )"

SUMMARY OF INVENTION

Technical Problem

However, the binders of Patent Literatures 1 and 2 still have a problem in which sufficient adhesion is not provided. Furthermore, Patent Literature 1 describes blending two or more types of polyvinylidene fluoride containing a copolymer according to Patent Literature 1, but does not describe a specific mixture formulation.
Furthermore, the copolymer of acrylic acid and vinylidene fluoride according to Patent Literature 1 has problems where a copolymer or an aggregate of a copolymer and electrode active materials is formed on an electrode surface when preparing an electrode, causing a reduction in battery properties.
In view of the foregoing, an object of the present invention is to provide a binder composition having high adhesion, without producing a copolymer or an aggregate of a copolymer and electrode active materials.

Solution to Problem

As a result of extensive studies on a configuration of a binder composition, the present inventors discovered that high adhesion can be achieved while suppressing the occurrences of an aggregate of a copolymer, by blending or combining polyvinylidene fluoride having an inherent viscosity at a certain value or higher and polyvinylidene fluoride containing acrylic acid or methacrylic acid as a monomer unit, as a component of the binder composition. In other words, the present invention can be expressed as follows.
A binder composition according to the present invention is a binder composition used for binding electrode active materials to a current collector where the electrode active materials are coated, containing: a first vinylidene fluoride polymer with an inherent viscosity of 1.7 dL/g or higher; and a second vinylidene fluoride polymer containing acrylic acid or methacrylic acid as a monomer unit.

Advantageous Effects of Invention

The present invention can provide a binder composition having high adhesion, without producing a copolymer or an aggregate of a copolymer and electrode active materials. Furthermore, an effect is achieved where the discharge capacity can be prevented from greatly being reduced, caused by peeling of an electrode mixture layer peeling from an aggregate during battery use, based on high adhesion between the current collector and an electrode mixture layer.

Brief Description of the Drawings

FIG. 1 is a cross-sectional view of an electrode in a non-aqueous electrolyte secondary battery according to the present embodiment.
FIG. 2 is an exploded perspective view of the non-aqueous electrolyte secondary battery according to the present embodiment.
FIGS. 3A and 3B are views of images illustrating a surface of an electrode manufactured in the present examples, where FIG. 3A illustrates an electrode surface for Example 3, and FIG. 3B illustrates an electrode surface for Comparative Example 10.

Description of Embodiments

An embodiment of the present invention is described in detail below. Herein, unless otherwise specified, "electrode" in the present specification and the like refers to an electrode of a non-aqueous electrolyte secondary battery, where an electrode mixture layer formed from an electrode mixture used in a binder composition in the present embodiment is formed on a current collector. Furthermore, "battery" in the present specification and the like refers to a non-aqueous electrolyte secondary battery provided with the "electrode". Furthermore, "adhesion" in the present specification and the like refers to adhesion between the current collector and electrode mixture layer formed on the current collector, and can be expressed by the peel strength of the electrode mixture layer. In other words, as the peel strength of the electrode mixture layer increases, the "adhesion" can be said to be more favorable.

Binder Composition

A binder composition according to the present embodiment is used for binding electrode active materials to a current collector, on an electrode provided by a battery, formed by forming an electrode mixture layer containing the electrode active materials on the current collector. Two types of a vinylidene fluoride polymer are included in the binder composition. For the sake of convenience, in the present specification and the like, the two types are referred to as a "first vinylidene fluoride polymer" and "second vinylidene fluoride polymer". Furthermore, in the present specification and the like, a "mixture of the first vinylidene fluoride polymer and second vinylidene fluoride polymer" is also referred to as "blended material". Note that the binder composition according to the present embodiment may further include another polymer may so long as a desired effect is not inhibited.
The first vinylidene fluoride polymer and second vinylidene fluoride polymer are described below in detail.

First Vinylidene Fluoride Polymer

The first vinylidene fluoride polymer is vinylidene fluoride having an inherent viscosity of 1.7 dL/g or higher.
"Vinylidene fluoride polymer" in the present specification includes either a homopolymer of vinylidene fluoride or copolymer of a vinylidene fluoride and a monomer that is copolymerizable with vinylidene fluoride. The monomer that is copolymerizable with a vinylidene fluoride is not particularly limited so long as the monomer is conventional at the time of the present application. If copolymerizing vinylidene fluoride, 90 mol% or more of a vinylidene fluoride unit is preferably included, and 95 mol% or more of the vinylidene fluoride unit is particularly preferably included.
Of these, the first vinylidene fluoride polymer is preferably a vinylidene fluoride homopolymer (PVDF).
The inherent viscosity of the first vinylidene fluoride polymer is 1.7 dL/g or higher and preferably 2.1 dL/g or higher. When the inherent viscosity is 1.7 dL/g or higher, a high adhesive performance can be achieved.
In other words, the first vinylidene fluoride polymer is particularly preferably a vinylidene fluoride homopolymer, with an inherent viscosity of 2.1 or higher.
The first vinylidene fluoride polymer can be manufactured by a conventionally known manufacturing method for manufacturing a vinylidene fluoride polymer. In other words, so long as the manufacturing method is appropriately set such that the aforementioned inherent viscosity is provided, the manufacturing method is not particularly limited.

Second Vinylidene Fluoride Polymer

The second vinylidene fluoride polymer is a vinylidene fluoride polymer containing (meth)acrylic acid as a monomer unit. The second vinylidene fluoride polymer preferably contains (meth)acrylic acid in addition to vinylidene fluoride as a monomer unit, and other monomer units may be included. Furthermore, monomer units of both acrylic acid and methacrylic acid may be included. Of these, a copolymer of (meth)acrylic acid and vinylidene fluoride is preferable. Of these, a copolymer of acrylic acid and vinylidene fluoride is more preferable. Note that "(meth)acrylic acid" in the present specification refers to either acrylic acid or methacrylic acid.
The inherent viscosity of the second vinylidene fluoride polymer is preferably 1.0 dL/g or higher, and more preferably 1.3 dL/g or higher.
The second vinylidene fluoride polymer is obtained by continuously adding (meth)acrylic acid or an aqueous solution containing (meth)acrylic acid to vinylidene fluoride, and then copolymerizing. While continuing a copolymerization reaction, continuous supplying of the (meth)acrylic acid) or aqueous solution of (meth)acrylic acid is preferably continued. Examples of a copolymerizing method can include suspension polymerization, emulsion polymerization, solution polymerization, and other conventionally known methods. Of these, from the perspective of ease of post-treatment and the like, suspension polymerization of an aqueous system and emulsion polymerization are preferable, and suspension polymerization of an aqueous system is more preferable as the copolymerizing method.
Examples of a suspending agent in suspension polymerization using water as a dispersing medium can include methylcelluloses, methoxylated methylcelluloses, propoxylated methylcelluloses, hydroxyethyl celluloses, hydroxypropyl celluloses, polyvinyl alcohols, polyethylene oxides, gelatins, and the like.
Furthermore, examples of a polymerization initiator can include diisopropyl peroxycarbonate, di-normal propyl peroxydicarbonate, di-normal heptafluoropropyl peroxydicarbonate, isobutyryl peroxide, di(chlorofluoroacyl) peroxide, di(perfluoroacyl) peroxide, and the like.
Furthermore, ethyl acetate, methyl acetate, acetone, ethanol, n-propanol, acetaldehyde, propyl aldehyde ethyl propionate, carbon tetrachloride, or other chain transfer agent can be added to adjust the degree of polymerization of an obtained polymer.

Mixture of First Vinylidene Fluoride Polymer and Second Vinylidene Fluoride Polymer

The binder composition according to the present embodiment is obtained by mixing the first vinylidene fluoride polymer and second vinylidene fluoride polymer. A ratio of a mixing amount of the first vinylidene fluoride polymer and mixing amount of the second vinylidene fluoride polymer of the binder composition is preferably 75:25 to 25:75 by weight ratio, more preferably 70:30 to 30:70, and even more preferably 60:40 to 40:60.
If the ratio of a mixing amount of the first vinylidene fluoride polymer and second vinylidene fluoride polymer in the binder composition is 1:1, the inherent viscosity of the first vinylidene fluoride polymer is more preferably within a range of 2.1 to 3.1 dL/g, and even more preferably within a range of 2.1 dL/g.

Advantages of Binder Composition

The binder composition according to the present embodiment can achieve high adhesion between an electrode mixture layer and current collector, in other words, high peel strength on an electrode having an electrode mixture layer formed from an electrode mixture using the binder composition according to the present embodiment. Furthermore, with an electrode having an electrode mixture layer obtained using the binder composition according to the present invention, an aggregate of a copolymer can be suppressed from occurring on an electrode surface.
Note that in the present specification, "a copolymer or an aggregate of a copolymer and electrode active materials is suppressed from occurring" indicates a case where a manufactured electrode is cut to 2 cm × 2 cm to prepare four pieces, with less than an average of three aggregates with a diameter of 1 mm or more or length of 1 mm or more per piece within the cut electrode surface area, or less than an average of five aggregates with a diameter of 0.5 mm or more or length of 0.5 mm or more per piece.
In the binder composition according to the present embodiment, a hydrogen bond is formed at an interface between the current collector and (meth)acrylic acid included in the second vinylidene fluoride polymer, and therefore, adhesion is assumed to improve, but a principle for the binder composition in the present embodiment achieving the effect is not limited thereto.
Furthermore, when the mixture ratio of the first vinylidene fluoride polymer and second vinylidene fluoride polymer is within a range of 75:25 to 25:75, further improvement of adhesion between the current collector and electrode mixture layer can be achieved on the electrode having the electrode mixture layer formed from the electrode mixture using the binder composition.
Furthermore, when the mixture ratio of the first vinylidene fluoride polymer and second vinylidene fluoride polymer is 1:1, the first vinylidene fluoride polymer with an inherent viscosity within a range of 2.1 to 3.1 dL/g is used, and therefore, adhesion between the current collector and electrode mixture layer can be further improved.

Electrode Mixture

The electrode mixture in the present embodiment contains electrode active materials and non-aqueous solvent in the binder composition. The electrode mixture layer is formed by coating the electrode mixture on the current collector to prepare the electrode. The electrode mixture is a slurry, and can be adjusted to a desired viscosity by adjusting the amount of the non-aqueous solvent.
The electrode mixture can be an electrode mixture for a cathode or electrode mixture for an anode by changing the type of electrode active materials or the like based on the type of the current collector or the like as a coating target. The electrode mixture in the present embodiment is preferably an electrode mixture for a cathode using an electrode active material for a cathode, in other words, a cathode active material (cathode material).

Non-aqueous Solvent

The non-aqueous solvent used in the electrode mixture in the present embodiment is not particularly limited so long as the solvent can dissolve the polyvinylidene fluoride. Examples of the non-aqueous solvent include N-methyl-2-pyrrolidone (NMP), dimethylformamide, N,N-dimethyl acetamide, N,N-dimethyl sulfoxide, hexamethyl phosphoramide, dioxane, tetrahydrofuran, tetramethylurea, triethyl phosphate, trimethyl phosphate, acetone, methyl ethyl ketone, tetrahydrofuran, and the like. The non-aqueous solvents may be used independently or use as a mixed solvent mixing two or more types thereof. Of these, the non-aqueous solvent used in the electrode mixture is preferably N-methyl-2-pyrrolidone, N,N,N-dimethylformamide, N-dimethylacetamide, or other organic solvent containing nitrogen, and is more preferably N-methyl-2-pyrrolidone.
When the total amount of the first vinylidene fluoride polymer and second vinylidene fluoride polymer is 100 parts by mass, the amount of the non-aqueous solvent is preferably 400 to 10,000 parts by mass, and more preferably 600 to 5,000 parts by mass. When the amount of the non-aqueous solvent is within the aforementioned range, the solution has an appropriate viscosity, and handling properties are excellent.

Electrode Active Materials

Electrode active materials used in the electrode mixture in the present embodiment may be an electrode active material for an anode, in other words, an anode active material if the electrode mixture according to the present embodiment is an electrode mixture for an anode, or may be an electrode active material for a cathode, in other words, a cathode active material if the electrode mixture according to the present embodiment is an electrode mixture for a cathode.
Examples of cathode active materials include lithium-based cathode active materials containing lithium. Examples of lithium-based cathode active materials include: composite metal chalcogen compounds as expressed by general formula LiMY₂ such as LiCoO₂, LiCo_xNi_1-xO₂ (0 ≤ x < 1), and the like; composite metal oxides; composite metal oxides having a spinel structure such as LiMn₂O₄ and the like; LiFePO₄ and other olivine type lithium compounds; and the like. Herein, M represents at least one type of transition metal such as Co, Ni, Fe, Mn, Cr, V, or the like, and Y represents a chalcogen element such as O, S, or the like.
Anode active materials can be a conventional known material including graphite and other carbon materials.
In the present embodiment, electrode active materials are preferably directly added to the blended material. Alternatively, electrode active materials may be first added to the non-aqueous solvent, and then the stirred and mixed product may be added to the blended material.

Conductive Additives

The electrode mixture in the present embodiment may further contain conductive additives. If an active material with low electrical conductivity such as LiCoO₂ is used, conductive additives are added with the objective of improving the conductivity of the electrode mixture layer. Examples of conductive additives can include carbon black, carbon nanotubes, graphite fine powder, graphite fiber, and other carbon materials, nickel, aluminum, and other metal fine powders or metal fibers.

Other Components of Electrode Mixture

The electrode mixture in the present embodiment may contain another component other than the aforementioned components. Examples of another component can include polyvinyl pyrrolidones, other pigment dispersants, and the like.

Electrode for Non-aqueous Electrolyte Secondary Battery

The electrode according to the present embodiment is described below while referring to FIG. 1. FIG. 1 is a cross-sectional view of the electrode of the present embodiment. As illustrated in FIG. 1, an electrode 10 has a current collector 11 and electrode mixture layers 12a and 12b, and the electrode mixture layers 12a and 12b are formed on the current collector 11. As described above, the electrode 10 is a cathode if the electrode mixture layers 12a and 12b are obtained using an electrode mixture for a cathode, and is an anode if the electrode mixture layers 12a and 12b are obtained using an electrode mixture for an anode.
The current collector 11 is a substrate of the electrode 10 and is a terminal for removing electricity. Examples of the material of the current collector 11 can include iron, stainless steel, steel, copper, aluminum, nickel, titanium, and the like. A form of the current collector 11 is preferably a foil or net. If the electrode 10 is a cathode, the current collector 11 is preferably an aluminum foil. The thickness of the current collector 11 is preferably 5 to 100 µm, and more preferably 5 to 20 µm. If the electrode 10 is small in size, the thickness of the current collector 11 may be 5 to 20 µm.
The electrode mixture layers 12a and 12b are layers obtained by coating the aforementioned electrode mixture on the current collector 11 and then drying. A conventionally known method in the technical field can be used as a method of coating the electrode mixture, and examples can include methods using a bar coater, die coater, comma coater, or the like. The drying temperature for forming the electrode mixture layers 12a and 12b is preferably 50 to 170°C. Furthermore, the thickness of the electrode mixture layers 12a and 12b is preferably 10 to 1000 µm. Note that the electrode 10 in FIG. 1 has the electrode mixture layers 12a and 12b formed on both surfaces of the current collector 11, but is naturally not limited thereto, and the electrode mixture layer may be formed on only one surface of the current collector 11.
The thickness of the electrode mixture layer is normally 20 to 250 µm, and preferably 20 to 150 µm. Furthermore, the basis weight of the mixture layer normally is 20 to 700 g/m², and preferably 30 to 500 g/m².

Non-aqueous Electrolyte Secondary Battery

A battery according to the present embodiment is described below while referring to FIG. 2. FIG. 2 is an exploded perspective view of a non-aqueous electrolyte secondary battery. A battery 100 has a cathode 1, anode 2, separator 3, and metal casing 5. Specifically, the battery 100 has a structure where a power generating element with a laminate body disposed on the separator 3 between the cathode 1 and anode 2 wound into a spiral shape is stored in the metal casing 5. Herein, the cathode 1 and anode 2 are the same as the electrode 10 in FIG. 1. A conventional material such as a porous film of polypropylene, polyethylene, or other polymer material or the like can be used for the separator 3.
Note that in FIG. 2, the battery 100 is illustrated as a cylindrical battery, but the battery 100 in the present embodiment is not limited thereto, and may be coin shaped, square shaped, or paper shaped.
Embodiments of the present invention will be described in further detail below using examples. The present invention is not limited to the following examples, and it goes without saying that various aspects are possible with regard to the details thereof. Furthermore, the present invention is not limited to the aforementioned embodiments, and various modifications are possible within the scope indicated in the claims. Embodiments obtained by appropriately combining disclosed technical means are also included in the technical scope of the present invention. Furthermore, all of the documents described in the present specification are hereby incorporated by reference.

Summary

A binder composition according to the present invention is a binder composition used for binding electrode active materials to a current collector where the electrode active materials are coated, containing: a first vinylidene fluoride polymer with an inherent viscosity of 1.7 dL/g or higher; and a second vinylidene fluoride polymer containing (meth)acrylic acid as a monomer unit.
Furthermore, in the binder composition according to the present invention, the inherent viscosity of the second vinylidene fluoride polymer is preferably 1.0 dL/g.
Furthermore, in the binder composition according to the present invention, the first vinylidene fluoride polymer is preferably a polymer containing only vinylidene fluoride as a monomer unit.
Furthermore, in the binder composition according to the present invention, the inherent viscosity of the first vinylidene fluoride polymer is preferably 2.1 dL/g.
Furthermore, in the binder composition according to the present invention, the mixture ratio of the first vinylidene fluoride polymer and second vinylidene fluoride polymer is preferably 75:25 to 25:75 by weight ratio.
Furthermore, in the binder composition according to the present invention, a non-aqueous solvent and the aforementioned electrode active material is preferably included.
Furthermore, in the binder composition according to the present invention, the electrode active material is preferably a cathode material.
An electrode for a non-aqueous electrolyte secondary battery according to the present invention is an electrode for a non-aqueous electrolyte secondary battery having a current collector and an electrode mixture layer formed on the current collector, and is an electrode for a non-aqueous electrolyte secondary battery where the aforementioned electrode mixture layer is a layer prepared using the aforementioned binder composition.
The non-aqueous electrolyte secondary battery according to the present invention is a non-aqueous electrolyte secondary battery provided with the aforementioned electrode for a non-aqueous electrolyte secondary battery.

Examples

As described below, electrodes were manufactured using various binder compositions according to the present invention, and a peel test was performed using the electrodes. Note that before describing specific examples, a method of calculating an "inherent viscosity" in the present specification will be described below.

Inherent Viscosity η_i

In order to calculate an inherent viscosity η_i, 80 mg of a polymer was dissolved in 20 mL of N,N-dimethylformamide to prepare a polymer solution. A viscosity η of the polymer solution is measured using an Ubbelohde viscometer in a 30°C constant temperature tank. The inherent viscosity η_i is determined by the following equation using the viscosity η. $η_{i} = (1 / C) \cdot \ln (η / η_{0})$
In the equation, n₀ represents a viscosity of N,N-dimethylformamide serving as a solvent, and C represents 0.4 g/dL.

Observation on Electrode Surface

Next, a method of observing a surface of the obtained electrode will be described below. A manufactured electrode is cut to 2 cm × 2 cm, and the number of aggregates with a diameter of 1 mm or more or length of 1 mm or more and aggregates with a diameter of 0.5 mm or more or length of 0.5 mm or more within a cut electrode surface range was observed. The number was determined by visually observing four cut electrode pieces, and calculating the number of present aggregates per piece. With regard to the presence or absence of aggregates, aggregates of copolymers are determined to be present if there are an average of three or more aggregates of vinylidene fluoride with a diameter of 1 mm or more or length of 1 mm or more on the surface of each electrode, or an average of five or more aggregates of vinylidene fluoride with a diameter of 0.5 mm or more or length of 0.5 mm or more on the surface of each electrode.

Example 1

First Vinylidene Fluoride Polymer: Vinylidene Fluoride Homopolymer (PDF) PVDF (KF #1700 manufactured by Kureha Corporation) with an inherent viscosity of 1.7 dL/g was used as the first vinylidene fluoride polymer.

Second Vinylidene Fluoride Polymer: Vinylidene Fluoride/Acrylic Acid Copolymer (VDF/AA Copolymer)

900 g of ion exchanged water, 0.4 g of hydroxypropyl methylcellulose, 2 g of butyl peroxypivalate, 396 g of vinylidene fluoride, and 0.2 g of initially added acrylic acid were incorporated in an autoclave with a 2 L capacity, and then heated to 50°C. A 1 wt.% acrylic acid aqueous solution containing acrylic acid was continuously supplied to a reaction container in a condition where constant pressure was maintained during polymerization. The obtained polymer slurry was dewatered and dried, and thus a VDF/AA copolymer was obtained as the second vinylidene fluoride polymer. The acrylic acid was added at a total amount of 4 g including the initially added amount. The inherent viscosity of the obtained VDF/AA copolymer was 2.5 dL/g.

Manufacturing of Electrode Mixture

PVDF was dissolved in N-methyl-2 pyrrolidone (hereinafter, NMP) to prepare a 5 wt.% concentration vinylidene fluoride polymer solution. A 5 wt.% concentration solution of the VDF/AA copolymer was also prepared by a similar method.
The two obtained solutions were mixed such that a mixture ratio of the PVDF and VDF/AA copolymer was a ratio of 50:50, stirred at 25°C, and then homogenized to prepare a 5 wt.% concentration binder mixed solution.
The 5 wt.% concentration binder mixed solution containing 2 parts by weight of a binder composition with regard to 100 parts by weight of LFP (LFP; LiFePO₄, average particle size: 1.2 µm, specific surface area: 14.7 m²/g) as an electrode active material is mixed for one minute, and then mixed for five minutes after adding N-methyl-2-pyrrolidone to obtain an electrode mixture where the total solid content concentration of the binder composition and electrode active material is 47 wt.%.

Electrode Manufacturing 1

The obtained electrode mixture is coated by a bar coater on a 15 µm thick aluminum foil serving as a current collector, primary dried for 30 minutes at 110°C in a nitrogen atmosphere, and then secondary dried for 2 hours at 130°C in a nitrogen atmosphere to prepare an electrode with a dry mixture basis weight of approximately 150 g/m².

Example 2

An electrode was prepared similarly to Example 1, other than the first vinylidene fluoride polymer was changed to PVDF (KF #7200 manufactured by Kureha Corporation) with an inherent viscosity of 2.1 dL/g.

Example 3

An electrode was prepared similarly to Example 1, other than the first vinylidene fluoride polymer was changed to PVDF (KF #7300 manufactured by Kureha Corporation) with an inherent viscosity of 3.1 dL/g.

Example 4

An electrode was prepared similarly to Example 3, other than the ratio of the PVDF and VDF/AA copolymer was set to 25:75.

Example 5

An electrode was prepared similarly to Example 3, other than the ratio of the PVDF and VDF/AA copolymer was set to 75:25.

Example 6

An electrode was prepared similarly to Example 1, other than the PVDF (KF #4300 manufactured by Kureha Corporation) with an inherent viscosity of 3.1 dL/g was used as the first vinylidene fluoride polymer, the ratio of the PVDF and VDF/AA copolymer was set to 3:2, and 2 parts by weight of carbon black (SP; SuperP (registered trademark) Li manufactured by Timcal Japan, average particle size: 40 nm, specific surface area 60 was added to the electrode active material as a conductive additive when preparing the electrode mixture.

Example 7

An electrode was prepared to similarly to Example 6, other than carbon nanotubes (CNT; average diameter: 15 nm, specific surface area: 200 were used as a conductive additive in place of SP.

Example 8

An electrode was prepared similarly to Example 3, other than the amount of butyl peroxypivalate and the initial added amount of acrylic acid was changed to 6 g and 0.8 g respectively, when preparing the second vinylidene fluoride polymer. The inherent viscosity of the obtained VDF/AA copolymer was 1.5 dL/g.

Example 9

An electrode was prepared similarly to Example 3, other than the initial added amount of acrylic acid was set to 0.8 g when preparing the second vinylidene fluoride polymer. The inherent viscosity of the VDF/AA copolymer obtained at this time was 3.0 dL/g.

Comparative Example 1

An electrode was prepared similarly to Example 1, other than the first vinylidene fluoride polymer was changed to PVDF (KF #1100 manufactured by Kureha Corporation) with an inherent viscosity of 1.1 dL/g.

Comparative Example 2

An electrode was prepared similarly to Example 1, other than only the PVDF (KF #7300 manufactured by Kureha Corporation) with an inherent viscosity of 3.1 dL/g was used without using the second vinylidene fluoride polymer, and 2 parts by weight of SP was added to the electrode active material as a conductive additive when preparing the electrode mixture.

Comparative Example 3

An electrode was prepared similarly to Example 1, other than only the VDF/AA copolymer was used without using the first vinylidene fluoride polymer, and 2 parts by weight of SP was added to the electrode active material as a conductive additive when preparing the electrode mixture.

Comparative Example 4

An electrode was prepared similary to Comparative Example 2, other than CNT was used in place of SP as a conductive additive.

Comparative Example 5

An electrode was prepared similary to Comparative Example 3, other than CNT was used in place of SP as a conductive additive.

Comparative Example 6

An electrode was prepared similarly to Example 3, other than the VDF/AA copolymer was replaced by a vinylidene fluoride polymer containing a carboxyl group with an inherent viscosity of 2.1 dL/g.
For the vinylidene fluoride polymer containing a carboxyl group, 1040 g of ion exchanged water, 0.8 g of methylcellulose, 2 g of diisopropyl peroxydicarbonate, 396 g of vinylidene fluoride, and 4 g of a maleic acid monomethyl ester (vinylidene fluoride: maleic acid monoethyl ester = 100:1.01) were added to an autoclave with a 2 liter capacity, and then suspension polymerized at 28°C. After polymerization is completed, the polymer slurry is dewatered, the dewatered polymer slurry is washed with water, the polymer slurry is dewatered again and then dried for 20 hours for 80°C to obtain a vinylidene fluoride polymer containing a carboxyl group.

Comparative Example 7

An electrode was prepared similarly to Example 1, other than using only PVDF (KF #1700 manufactured by Kureha Corporation) with an inherent viscosity of 1.7 dL/g, and not using the second vinylidene fluoride polymer, and then the peel strength was measured.

Comparative Example 8

An electrode was prepared similarly to Example 1, other than using only PVDF (KF #7200 manufactured by Kureha Corporation) with an inherent viscosity of 2.1 dL/g, and not using the second vinylidene fluoride polymer.

Comparative Example 9

An electrode was prepared similarly to Example 1, other than using only PVDF (KF #7300 manufactured by Kureha Corporation) with an inherent viscosity of 3.1 dL/g, and not using the second vinylidene fluoride polymer.

Comparative Example 10

An electrode was prepared similarly to Example 1, other than using only the VDF/AA copolymer with an inherent viscosity of 2.5 dL/g, and not using the first vinylidene fluoride polymer.

Comparative Example 11

An electrode was prepared similarly to Example 1, other than using only the VDF/AA copolymer with an inherent viscosity of 1.5 dL/g prepared in Example 8 without using the first vinylidene fluoride polymer.

Comparative Example 12

An electrode was prepared similarly to Example 1, other than using only the VDF/AA copolymer with an inherent viscosity of 3.0 dL/g prepared in Example 9 without using the first vinylidene fluoride polymer.

Evaluation of Adhesion of Electrode Mixture Layer on Electrode Structure and Generation of Aggregates 1

Adhesion between the electrode mixture layer and aluminum foil on the electrodes obtained in Examples 1 to 7 and Comparative Examples 1 to 6 was evaluated as 90° peel strength at a head speed of 10 mm/minute by adhering an upper surface of the electrode formed by coating onto a thick plastic plate, and using a tensile testing machine ("STA-1150 UNIVERSAL TESTING MACHINE" manufactured by ORIENTEC) in accordance with JIS K6854-1. The thick plastic plate is made from an acrylic resin and is 5 mm thick. The peel strength of the Examples and Comparative Examples were measured. Furthermore, the generation of aggregates in the electrode mixture layer of the Examples and Comparative Examples were observed. The results are shown in Table 1.

Furthermore, in order to calculate a peel strength calculated value, the peel strength was measured similarly to Comparative Examples 7 to 12. Herein, the peel strength calculated value is a value of peel strength theoretically predicted when mixing the first vinylidene fluoride polymer and second vinylidene fluoride polymer, obtained by adding a value where a value of peel strength when the first vinylidene fluoride polymer and second vinylidene fluoride polymer are independently used is calculated based on the mixed amounts thereof. In other words, a value that is higher than the peel strength calculated value indicates that a synergistic effect occurs by mixing the two types of vinylidene fluoride polymers, as compared to independently using the polymers. The peel strength in Comparative Examples 7, 8, 9, 10, 11, and 12 was 0.19 gf/mm, 0.19 gf/mm, 0.17 gf/mm, 1.28 gf/mm, 0.54 gf/mm, and 0.91 gf/mm.

Table 1

	Additive	First Vinylidene Fluoride Polymer		Second Vinylidene Fluoride Polymer		Peel Strength	Peel Strength Measured Value [gf/mm]	Aggregate
	Additive	Inherent Viscosity [dl/g]	Mixed amount [wt%]	Inherent Viscosity [dl/g]	Mixed amount [wt%]	[gf/mm]	Peel Strength Measured Value [gf/mm]
Example 1	-	1.7	50	2.5	50	1.35	0.73	No
Example 2	-	2.1	50	2.5	50	1.71	0.74	No
Example 3	-	3.1	50	2.5	50	1.46	0.73	No
Example 4	-	3.1	25	2.5	75	0.62	0.45	No
Example 5	-	3.1	75	2.5	25	1.26	1.00	No
Comparative example 1	-	1.1	50	2.5	50	0.58	0.73	No
Comparative example 6	-	3.1	50	2.1	50	0.85	0.97	No
Example 6	SP	3.1	60	2.5	40	1.98	0.60	No
Comparative example 2	SP	3.1	100	-	-	0.20	-	No
Comparative example 3	SP	-	-	2.5	100	1.19	-	Yes
Example 7	CNT	3.1	60	2.5	40	2.38	0.74	No
Comparative example 4	CNT	3.1	100	-	-	0.00	-	No
Comparative example 5	CNT	-	-	2.5	100	1.86	-	Yes
Example 8	-	3.1	50	1.5	50	1.63	0.36	No
Example 9	-	3.1	50	3.0	50	1.29	0.54	No

As shown in Table 1, Examples 1 to 5 demonstrated high peel strength as compared to Comparative Example 1 in conjunction with demonstrating higher peel strength than the peel strength calculated value. In particular, Example 2 demonstrated a peel strength of approximately 2.9 time that of Comparative Example 2. The peel strength in Comparative Example 1 is lower than the peel strength calculated value, and an effect of improving the peeling strength by mixing was not observed. Of these, Examples 1 to 3 demonstrated higher peel strength than when the vinylidene fluoride polymers are independently used.
For Examples 6 and 7 using a conductive additive, a higher peel strength than the peel strength calculated value was demonstrated, and a higher peeling strength than when the vinylidene fluoride polymers were independently used was demonstrated.
Comparative Example 6 using a vinylidene fluoride polymer containing a carboxyl group as the second vinylidene fluoride polymer is different from Example 3 where only the type of the second vinylidene fluoride polymer is different, and an effect of improving the peel strength by mixing was not improved.
Furthermore, in Examples 8 and 9 using vinylidene fluoride polymers with inherent viscosities of 1.5 dL/g and 3.0 dL/g as the second vinylidene fluoride polymer, a higher peel strength than the peel strength calculated value was demonstrated even with different inherent viscosities, and a higher peel strength than when independently using the vinylidene fluoride polymers was demonstrated.
Furthermore, the occurrence of aggregates was not observed in any of the Examples. Of these, as a representation, FIG. 3A and FIG. 3A respectively illustrate a photograph of an electrode surface of Example 3 and a photograph of an electrode surface of Comparative Example 10. As illustrated in FIG. 3B, aggregates were confirmed on the electrode surface of Comparative Example 10, and a copolymer of aggregates of a copolymer and electrode active material occurred. On the other hand, as illustrated in FIG. 3A, the presence of a copolymer or aggregates of a copolymer and electrode active materials could not be confirmed in Example 3.

Electrode Manufacture 2

Example 10

An electrode was prepared similarly to Example 6, other than a lithium-nickel-cobalt-manganese composite oxide (NCM111; Li_1.00Ni_0.33Co_0.33Mn_0.33O₂, Average particle size: 6 µm) was used as the electrode active material.

Comparative Example 13

An electrode was prepared similarly to Comparative Example 2, other than NCM111 (Li_1.00Ni_0.33Co_0.33Mn_0.33O₂, Average particle size: 6 µm) was used as the electrode active material.

Comparative Example 14

An electrode was prepared similarly to Comparative Example 3, other than NCM111 (Li_1.00Ni_0.33Co_0.33Mn_0.33O₂, Average particle size: 6 µm) was used as the electrode active material.

Example 11

An electrode was prepared similarly to Example 6, other than a lithium-cobalt composite oxide (LCO; LiCoO₂, Cell seed C5H manufactured by Nippon Chemical Industrial Co., Ltd., Average particle size: 5 µm) was used as the electrode active material.

Comparative Example 15

An electrode was prepared similarly to Comparative Example 2, other than LCO (LiCoO₂, Cell seed C5H manufactured by Nippon Chemical Industrial Co., Ltd., Average particle size: 5 µm) was used as the electrode active material.

Comparative Example 16

An electrode was prepared similarly to Comparative Example 3, other than LCO (LiCoO₂, Cell seed C5H manufactured by Nippon Chemical Industrial Co., Ltd., Average particle size: 5 µm) was used as the electrode active material.

Evaluation of Adhesion of Electrode Mixture Layer on Electrode Structure and Generation of Aggregates 2

Similar to the aforementioned "Evaluation of Adhesion of Electrode Mixture Layer On Electrode Structure and Generation of Aggregates 1", the peel strength of Examples 10 and 11 and Comparative Examples 13 to 16 were measured. Furthermore, the generation of aggregates in the electrode mixture layer were observed. The results are shown in Table 2.

[Table 2]

	Cathode Active Material	First Vinylidene Fluoride Polymer		Second Vinylidene Fluoride Polymer		Peel Strength	Peel Strength Measured Value [gf/mm]	Aggregate
	Cathode Active Material	Inherent Viscosity [dl/g]	Mixed amount [wt%]	Inherent Viscosity [dl/g]	Mixed amount [wt%]	[gf/mm]	Peel Strength Measured Value [gf/mm]
Example 10	NCM111	3.1	60	2.5	40	0.82	0.53	No
Comparative example 13	NCM111	3.1	100	-	-	0.31	-	No
Comparative example 14	NCM111	-	-	2.5	100	0.86	-	Yes
Example 11	LCO	3.1	60	2.5	40	0.90	0.58	No
Comparative example 15	LCO	3.1	100	-	-	0.34	-	No
Comparative example 16	LCO	-	-	2.5	100	0.82	-	Yes

As shown in Table 2, both Examples 10 and 11 demonstrated a higher peel strength than the peel strength calculated value, regardless of the type of cathode active material.

Industrial Applicability

The present invention can be used as a binder composition used in binding an electrode active material and current collector in a non-aqueous electrolyte secondary battery.

Reference Signs List

1 Cathode
2 Anode
3 Separator
5 Metal casing
10 Electrode
11 Current collector
12a Electrode mixture layer
12b Electrode mixture layer
100 Battery

Claims

A binder composition used for binding an electrode active material to a current collector where the electrode active material is coated, comprising:
a first vinylidene fluoride polymer with an inherent viscosity of 1.7 dL/g or higher; and

a second vinylidene fluoride polymer containing acrylic acid or methacrylic acid as a monomer unit.
The binder composition according to claim 1, wherein the inherent viscosity of the second vinylidene fluoride polymer is 1.0 dL/g or higher.
The binder composition according to claim 1 or 2, wherein the first vinylidene fluoride polymer contains only vinylidene fluoride as a monomer unit.
The binder composition according to any one of claims 1 to 3, wherein the inherent viscosity of the first vinylidene fluoride polymer is 2.1 dL/g or higher.
The binder composition according to any one of claims 1 to 4, wherein a mixture ratio of the first vinylidene fluoride polymer and second vinylidene fluoride polymer is 75:25 to 25:75 by weight ratio.
The binder composition according to any one of claims 1 to 5, comprising: a non-aqueous solvent; and
the electrode active material.
The binder composition according to claim 6, wherein the electrode active material is a cathode material.
An electrode for a non-aqueous electrolyte secondary battery, comprising:
a current collector; and
an electrode mixture layer formed on the current collector; wherein
the electrode mixture layer is a layer prepared using the binder composition according to any one of claims 1 to 7.
A non-aqueous electrolyte secondary battery, comprising the non-aqueous electrolyte secondary battery according to claim 8.